1. Field of the Invention
This invention relates generally to metallurgical furnaces having a heating shaft for use with sized materials, and to the formation of the internal surface of the preheat zone of the furnace for the control of gas flow during operation of the furnace. More specifically, the present invention relates to improvements in metallurgical furnaces whose charge is relatively uniformly sized metallic oxides and carbonaceous fuel or relatively uniformly sized metal oxides and lump carbonaceous fuel. It is particularly suited to furnaces used in the reduction of iron oxide.
2. Prior Art
Over the last several decades, the reserves of high-grade metallic lump ores have been largely depleted. Therefore, numerous processes have been developed to upgrade finely divided high-grade ores, and to enrich such low quality ores as taconite. More recently, these processes have been applied to metallic wastes, such as blast furnace dust, BOF dust, open-hearth dust, mill scale, and other such materials produced in iron and steel making processes. They are also being applied to low quality coals and coke dust. These processes are generally known as "beneficiation."
Common to most of these processes is the agglomeration procedure. During agglomeration, the finely divided material is compacted into stronger, larger, more dense particles suitable for charging into a shaft furnace. The agglomerating step may include pelletizing, briquetting, extrusion and sintering. Whatever the agglomeration procedure, the product is ideally composed of small, uniformly sized particles whose largest dimension is generally less than 2 inches. These particles are made from finely divided metal oxides whose size may vary from one-quarter inch to submicron in size, or from long metallic strands or chips produced during finishing or machining operations.
After agglomeration, the agglomerates are delivered to a refining process for final use. The present invention relates to those processes which employ a metallurgical furnace having a shaft preheating section for using a sized charge of burden, or agglomerates. The term "vertical preheating section" as used herein is defined as a container for the sized burden, higher than it is wide, and utilizing an upward flow of gas or fluid through the sized burden to accomplish either heating or partial reduction of the oxide or metal with subsequent melting in the same or another vessel.
Prior to the development of beneficiation, shaft furnaces were charged with lump material. Gas flow through the lumps was irregular, having a tendency to seek out the path of least resistance. Where such a lump charge was used, the void spaces between lumps were generally large enough so that the gases would find or make a tortuous path through the material and preheat the same relatively uniformly. With the use of relatively small and uniform sized burden in shaft furnaces, however, the void spaces between agglomerates are relatively small, as compared to those with lump material, and the gas permeability of the charge is significantly reduced. Under these conditions, the gases have the tendency to channel or to find direct paths between the charge and the walls of the shaft furnace. The present invention is designed to solve the problem of channeling at the periphery of shaft furnaces.
The formation of projections along the walls of furnaces has previously been proposed, such as is suggested in U.S. Pat. Nos. 3,379,427 and 3,843,106, but these projections and their purpose are not similar to those of the present invention. As suggested in U.S. Pat. No. 3,379,427, a blast furnace lining is proposed for the interior of the furnace, which lining, in the form of vertical metal plates, has projections and recesses to promote formation of a protective slag layer along water-cooled plates, but these plates to not form a continuous obstruction to gas flow. In U.S. Pat. No. 3,843,106, a furnace wall having coolers therein is proposed for use in an arc furnace, the purpose being to protect the walls from molten metal by formation of a protective layer of splash metal around the interior of the furnace bottom wall. Thus, these systems provide projections and recesses in the hot metal portions of the furnace to protect the furnace wall from molten slag and metal.